It would be helpful to add some hints in the official installation guide for Typo3 8.7 (does it already exist?) or in the error message itself. It is quite confusing when using a UTF8 database and getting an error it is not a UTF8 database.

TYPO3 requires your database and your tables to be utf-8 since at least 6.2, I think even 4.7. It may have worked by chance until now with a wrong charset but with TYPO3 8.7 you really should change this.

Your first all-inkl/strato screenshots just show the database connection you are using is utf8.

Your database may are may not be UTF-8, but neither on all-ink nor on strato your tables are UTF-8 as can be seen on the second screenshots you provided - all of them have a collation of latin1_swedish_ci.

Making sure your database has a collation of utf8_general_ci is only the first step. This secures tables newly created by TYPO3 have the correct collation. If you dump tables from another database you dump the collation information with it, so just inserting your latin1_swedish_ci tables from all-ink to the strato database does not make them utf8_general_ci

Convert all of your tables to utf8_general_ci and change the database collation to utf8_general_ci, if thats not already set.

Of course I know the UTF-8 is required for a long tim.e The funny thing is, that it worked at all-inkl with Version 7.6 and 8.7 without a warning in Install Tool. Only after moving to Strato the message occured.